Current conducting element for aluminum reduction cells



1965 c. E. RANSLEY 3,202,600

CURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTION CELLS Original FiledMay 23, 1957 8 Sheets-Sheet 1 a o o 9 I 00 o o co 0 o h m A o a z E 2 oa J 0 m a v 3 H :a 2

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t l o g \l N a U) g 3 3 '5 INVENTOR CHARLES ERIC RANSLEY ATTORNEY Aug.24, 1965 c. E. RANSLEY CURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTIONCELLS 8 Sheets-Sheet 2 Original Filed May 23, 1957 INVENTOR CHARLES ERICRANSLEY ATTORNEY Aug. 24, 1965 c. 1-: RANSLEY CURRENT CONDUCTING ELEMENTFOR ALUMINUM REDUCTION CELLS 8 Sheets-Sheet 5 Original Filed May 23,1957 u n 0 .1. .///U/ 1 n N M/ 0 H N 4 n T 6/: F

INVENTOR CHARLES ERIC RANSLEY ATTORNEY Aug. 24, 1965 c. E. RANSLEYCURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTION CELLS 8 Sheets-Sheet 4Original Filed May 23, 1957 INVENTOR CHARLE 5 ERIC RAN SLEY ATTORNEY BYWAug. 24, 1965 c. E. RANSLEY 3,202,600

CURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTION CELLS Original FiledMay 23, 1957 8 Sheets-Sheet 5 INVENTOR CHARLES ERIC RAN SLEY ATTORNEYAug. 24, 1965 c. E. RANSLEY 3,202,600

CURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTION CELLS 8 Sheets-Sheet 6Original Filed May 23, 1957 N MN// INVENTOR CHARLES ERIC RANSLEYATTORNEY Aug. 24, 1965 c. E. RANSLEY CURRENT CONDUCTING ELEMENT FORALUMINUM REDUCTION CELLS 8 Sheets-Sheet '7 Original Filed May 23, 1957INVENTOR CHARLES ERIC RANSLEY ATTORNEY Aug. 24, 1965 c. E. RANSLEYCURRENT CONDUCTING ELEMENT FOR ALUMINUM REDUCTION CELLS 8 Sheets-Sheet 8Original Filed May 23, 1957 INVENTOR CHARLES ERIC RANSLEY ATTORNEYUnited States Patent 3,202,600 CURRENT CONDUlITlNG ELEMENT FUR ALUMKNUMREDUCTl-QN CELLS Charles Eric Ransiey, Chesharn Rois, England, assignorto The British Aluminium (lumpany Limited, London, England, a companyoi. Great Britain ()riginal application May 23, 1957, Ser. No. 660,994,now Patent No. 3,028,324, dated Apr. 3, 1962. Divided and thisapplication .lune 12, 1961, Ser. No. 125,318 Claims priority,application Great Britain, May 4, 1951, 10,548/51, 10,549/5l; Aug. 3,1951, 18,490/51; Apr. '15, 1952, 9,474/5'2; Jan. 14, 1954, 1,154/54,1,155/54; Mar. 10, 1955, 7,1'3'5/55, 7,1'36/55, 7,137/55; May 1, 1957,13,948/57 13 Claims. (ill. 204-479) This application is a division ofapplication Serial No. 660,994, filed May 23, 1957 (now Patent No.3,028,324).

Application 660,994 is a continuation-in-part of my copendingapplications Serial Number 286,709, filed May 8, 1952; Serial Number481,611, filed January 13, 1955; Serial Number 481,927, filed January14, 1955; Serial Number 569,736 filed March 6, 1956; Serial Number569,737, filed March 6, 1956; Serial Number 570,233, filed March 8, 1956and Serial Number 613,006, filed October 1, 1956, said applicationSerial Number 613,006 being a continuation-impart of said Serial Numbers286,709; 481,611; 481,927; and 569,737 and application Serial Number284,761, filed April 28, 1952. Each of the above mentioned parentapplications except 660,994 is now abandoned.

This invention relates to electrolytic cell for the production ofmetals, e.g. aluminum. The expression electrolytic cell, as usedhereinafter, is meant in include both reduction cells for the productionof aluminum and three layer cells for the refining or purification ofaluminum. More particularly this invention is concerned withelectrolytic cells embodying one or more current-conducting elementswhich may constitute at least a part of the cathodes of reduction cellsor current-conducting elements for taking part in the supply ofelectrolyzing current to a body of molten metal and at least partiallyexposed to the latter either in a reduction cell or in a purificationcell. Additionally, this invention is concerned with electrolytic cellstructure involving the use of such currentconducting elements andmethods of operation of such cells.

Both of the above-mentioned cells, that is, the reduction cell and thethree-layer cell are of the kind in which it is necessary to passelectrolyzing current through a body of electrolyte or flux. In the caseof reduction cells the current passes between an anode and a cathodehaving their operative faces in contact with the body of electrolytewhich has dissolved therein a compound of the metal. The cathode may bethe pool of molten metal which collects on the floor of the cell or itmay be a solid electrode immersed at least partially in the electrolyte.Such an electrode may extend into the pool of molten metal in which casethe latter is also cathodic. In the case of three-layer purificationcells the current passes between the pool of aluminum alloy forming thebottom layer in the cell and the layer of purified molten aluminumforming the top layer in such a cell through the body of electrolyte orflux forming the intermediate layer which is in contact with both thetop and bottom layers. Hitherto no material has been found to have thechemical and electrical properties required of a solid electrodeconstituting the cathode in a reduction cell and further thearrangements at present made for leading the current into the body ofmolten metal in a reduction cell or the bodies of metal in a three-layercell are not entirely satisfactory and this is particularly true of thereduction cell where 3,202,600 Patented Aug. 24, 1965 relativelysubstantial losses in efliciency and increases in constructional andmaintenance costs are directly traceable to the nature of the currentleads which have to be employed.

For example, aluminum reduction cells which are at present in commercialuse are employed to effect the electrolysis of an aluminum compound,generally aluminum oxide, while it is dissolved in a suitable fi'uxwhich is mainly cryolite and has a fusion point usually in excess of 900C. Since the cells therefore must be operated at a temperature in theneighborhood of 1000 C., their construction has always presentedconsiderable problems. The flux, for example, is very reactive towardsmetals and towards normal refractory materials. Thus difiiculty isexperienced in constructing a durable receptacle or container for themolten flux and there is even greater difficulty in finding suitablematerials for the construction of a solid cathodic electrode in contactwith molten flux and molten metal and the current-conducting elementswhich are in contact with the molten metal.

Carbon is the only material which has hitherto been found to be capableof use for the purposes mentioned above, this being employed both forlining the receptacle which is to contain the electrolyte or fiux andfor the construction of the current-conducting elements. However, theuse of carbon entails a number of very considerable disadvantages, notthe least of which is the fact that the floor of the cell lining whichsupports the molten metal must, in practice, be arranged in asubstantially horizontal plane. With such arrangement the floor spaceoccupied by a single cell is quite extensive and the cost ofconstructing such large cells is considerable. The necessity for thishorizontal arrangement arises from the fact that molten aluminum doesnot wet carbon. Unless this arrangement be adopted the currentefiiciency of the cell is very low.

In the case of reduction cells, the use of carbon to conduct current tothe molten metal cathode and the horizontal arrangement of the cellfloor entail a number of disadvantages in operating the cell. Forexample, the gradual penetration of molten flux or flux constituentsinto the cell floor causes this to disintegrate and shortens its usefullife. Deposits are formed on the surface of the carbon which increasethe voltage drop across the cell and reduce the efiiciency of thelatter. Satisfactory electrical contact between the carbon lining andthe electrical current supply conductors, e.g. cathode collector bars,is difficult to achieve and there are appreciable losses also due to theelectrical resistance of the carbon itself.

The horizontal construction referred to hereinabove has the furtherdisadvantage that the inherent turbulence of the molten metal cathoderequires a high inter-polar distance to ensure against contact of themolten metal cathode with the anode and with the consequent productionof excess heat which has to be dissipated.

Accordingly, it is the primary purpose and object of this invention toprovide improved electrolytic cells for the production of aluminum, e.g.reduction cells and refining or purification cells, which overcome orsubstantially minimize the disadvantages as mentioned hereinabove.

Another object is to provide improved electrolytic cells for theproduction of aluminum wherein the cathode is solid.

Another object is to provide improved electrolytic cells for theproduction of aluminum, e.g. reduction and refining cells, wherein thecathodic electrical system involves the use of current-conductingelements giving rise to longer life and reduced voltage drop.

Another object is to provide novel current-conducting elements for usein electrolytic cells.

Another object is to provide methods for making current-conductingelements.

Another object is to provide a novel aluminum reduction cell involvingthe use of a solid cathode which is inclined.

Another object is to provide improved reduction and refining cells forthe production of aluminum wherein the cathodic electrical system doesnot involve, as an element therein, a carbon or graphite member.

Another object is to provide an improved method of operating cells forthe production of aluminum.

Another object is to provide an improved method of operating cells forthe production of aluminum involving the use of a sodiumchloride-containing electrolyte.

These and other objects and advantages of this invention will beapparent from the following description thereof taken in conjunctionwith the drawings wherein:

FIGURE 1 is a diagram showing the amount of titanium dissolved byaluminum at 970 C. in contact with materials ranging in composition from100% TiC to 100% TiB FIGURE 2 is a transverse vertical section of oneconstruction of electrolytic reduction cell as it appears duringoperation thereof, this section being taken on the line IIII of FIGURE3;

FIGURE 3 is a fragmentary section taken on the line IIIIII of FIGURE 2showing one end of the cell, the flux or electrolyte and molten aluminumbeing omitted for purpose of clarity;

FIGURE 4 is a view similar to that of FIGURE 2 showing an alternativeconstruction of reduction cell, this section being taken on the lineIVIV of FIGURE 5;

FIGURE 5 is a section taken on the line V-V of FIGURE 4, the contents ofthe cell, namely, the molten and solidified flux or electrolyte and thepool of molten metal, being omitted for purpose of clarity;

FIGURE 6 is a fragmentary section taken on the line VI-VI of FIGURE 4;

FIGURE 7 is a transverse vertical section of another construction ofelectrolic reduction cell as it appears during the operation thereof;

FIGURE 8 is a transverse fragmentary vertical section of anotherconstruction of electrolytic reduction cell as it appears during theoperation thereof;

FIGURE 9 is a transverse fragmentary vertical section of still a furtherconstruction of electrolyte reduction cell as it appears during theoperation thereof;

FIGURE 10 is a fragmentary longitudinal vertical sec tion taken on theline X-X of FIGURE 11 and showing a further construction of electrolyticreduction cell as it appears during the operation thereof;

FIGURE 11 is a fragmentary composite view of the left-hand end of thereduction cell shown in FIGURE 10, the upper part of the figure being asection taken on the line XIXI of FIGURE 10, and the lower part being aplan view with the body of flux in the layer of molten aluminum omittedfor purpose of clarity and part of one of the anodes broken away;

FIGURE 12 is a transverse vertical section similar to FIGURE 8illustrating a further modification of electrolytic reduction cell;

FIGURE 13 is a fragmentary plan view of the reduction cell shown inFIGURE 12, the flux and molten aluminum layer being omitted for purposeof clarity.

FIGURE 14 is a vertical section of a three-layer refining orpurification cell;

FIGURE 15 is a vertical section of a three-layer refining orpurification cell showing an alternative arrangement of the currentleads supplying the top layer;

FIGURE 16 is an elevational view, partially broken away, showing asheathed electrode in its position of use;

FIGURE 17 is a plan view of the electrode shown in FIGURE 16;

FIGURE 18 is a fragmentary elevational view, taken 4 from the right sideof FIGURE 16 and showing the upper part of the sheath; and

FIGURE 19 is a transverse vertical section of a threelayer refining orpurification cell employing the sheathed electrode shown in FIGURE 16.

With a view to overcoming or substantially minimizing the disadvantagesin prior art cells and cell operation, as mentioned hereinbefore, therehas been a long sought need for a new current-conducting element whichmay be exposed in part to molten aluminum, molten aluminumcontainingmetals, electrolyte or flux and temperatures and atmopheres incident tothe operation of electrolytic cells without material adverse effect. Ithas been concluded that the properties required in an idealcurrentconducting element may be summarized as follows:

(1) It shoulid have a good electrical conductivity.

(2) It must not react with nor be soluble in either molten aluminum or,under cathodic conditions, in molten fiux or electrolyte, at least toany appreciable extent, at the operating temperature of the cell. Thesolubility of the material in molten aluminum is an importantconsideration as it determines both the useful life of thecurrent-conducting element and the degree of contamination of thealuminum produced through the agency of such current-conductive element.

(3) It should be capable of being wetted by molten aluminum. Theimportance of wettability had not previously been recognized clearly,but my research demonstrated that immediate advantage would follow it amaterial with this property could be developed.

(4) It must be cheap enough to be fabricated in the required formeconomically.

(5) It should have high stability under the conditions existing at thecathode of the cell, that is, it should possess good resistance topenetration by the molten metal and to cracking.

(6)It should have a low thermal conductivy.

(7) It should have a good mechanical strength and resistance to thermalshock.

(8) Where the material of the current-conducting element is to beexposed to the exterior of the cell, it should have a good resistance tooxidation and to gases to which it is exposed. Normally the conditionsexisting at the cathode of a cell are highly reducing and in certainapplications therefor this requirement is not essential.

As the result of many experiments, it has been found that materialswhich exhibit all or substantially all of the properties hereina'boveset forth are the carbides and borides of titanium, zirconium, tantalumand niobium and mixtures thereof.

At least the operative face or faces of the current-conducting elements,i.e., the face or faces exposed to the deleterious conditions subsistingduring the operation of the cell, e.g., the face or faces exposed to themolten metal, may consist essentially of but one of the materialsspecified, or, alternatively, may consist essentially of more than onesuch material in varying proportions. In most applications, it ispreferred that the whole of the currentconducting elements shouldconsist essentially of one or more of such materials. The expressionconsist essentially, as used hereinafter in the specification andclaims, means that that portion of the element made of one or more ofthe carbides and borides referred to above does not contain thesubstances in amounts sutficient materially to affect the desirablecharacteristics of the current-conducting element although othersubstances may be present in minor amounts which do not materiallyaffect such desirable characteristics, for example small proportions ofoxygen or iron in titanium boride. It is also preferred that therefractory materials in the current-conducting elements be essentiallyfree from elements or compounds which would lead to undesirablecontamination of the aluminum produced. Nevertheless, thecurrent-conducting elements embodied in cells according to thisinvention may contain initially, among others, certain compounds whichdissolve out when the element is first put into service, and which donot materially affect the element.

Desirably, that portion of the element consisting essentially of one ormore of the above mentioned refractory materials should be composed ofat least 90% by weight of such material. The carbides and boridesreferred to have been found to possess a relatively high electricalconductivity (better than that of carbon), a good resistance to attackby molten flux or electrolyte, a very low solubility in molten aluminumat cell operating temperatures and a resistance to oxidationconsiderably better than that of carbon. They can be produced in asuitable form with good mechanical properties. Furthermore, it ispossible effectively to wet the surface of current-conducting elementsmade from these materials with molten aluminum under all operatingconditions, from which it results that, for the first time, acommercially practicable cell with vertical or inclined electrodes maybe constructed. The term cathode, as applied to solid members, isintended to denote an electrode of sheet, plate, rod or other suitableshape at which metal is produced in a tangible form. In addition, thesematerials can be connected without great difficulty to a metallicconductor to establish a good mechanical and electrical joint therewith.

Of the carbides and borides referred to above, those preferred at thepresent time are the compounds of titanium and zirconium since theelements tantalum and niobium are relatively rare and correspondinglyexpensive.

Accordingly, the present invention provides in an electrolytic cell forthe production of aluminum the combination comprising a receptacledefining a chamber adapted to contain a body of molten electrolyte and abody of molten aluminum and an electrical system including at least oneanodic current-conducting element extending into said chamber and atleast one cathodic current-conducting eleent extending into saidchamber, at least a part of the surface of one of saidcurrent-conducting elements being adapted to be exposed to moltenaluminum and consisting essentially of a material possessing a lowelectrical resistivity, a low solubility in molten aluminum and moltenelectrolyte under cell operating conditions and being wettable by moltenaluminum under cell operating conditions.

According to a further feature of the invention an electrolytic cell forthe production of aluminum comprises in combination a receptacledefining a chamber adapted to contain a body of molten electrolyte and abody of molten aluminum, an anode extending at least partially withinsaid chamber and a cathodic current-conducting element having at least apart of its surface exposed to the interior of said chamber and spacedfrom said anode, at least said part of said surface consistingessentially of a material possessing a low electrical resistivity, a lowsolubility in molten aluminum and molten electrolyte under celloperating conditions and being wettable by molten aluminum under celloperating conditions.

It is also a feature of the invention that said material eferred to inthe preceding two paragraphs has an e ectrical resistivity lower thancarbon and a solubility in molten aluminum and molten electrolyte undercell operating conditions at least as low as titanium carbide orzirconum carbide.

Advantageously said material is titanium boride.

A current-conducting element consisting essentially of one or more ofcarbides and borides referred to may be employed with advantage inaccordance with this invention to establish electrical connection withthe body of molten aluminum-containing metal in a cell of orthodoxconstruction, its use greatly reducing the voltage drop which wouldotherwise be experienced. Additionally, as mentioned above, such acurrent-conducting element may be used satisfactorily as a cathodedisposed in a vertical or inclined position. There are considerableadvantages to be gained by constructing a cell with a cathode orcathodes at least a part of the operative surface of which is sodisposed.

The cathode may be so arranged in the cell that its operative face orfaces, is or are, disposed at a relatively large angle, i.e., 60 to tothe horizontal, the deposited aluminum continuously draining down theface or faces concerned, preferably to collect in a pool in contact withthe lower part of the cathode from which pool it may be withdrawn fromtime to time in the usual manner. If desired, the pool of moltenaluminum thus formed may be utilized as part of the current-supply meansfor the cathode. The operative face or faces of the anode or anodes in acell embodying inclined cathodes, according to this invention, is or arealso disposed at a substantial angle to the horizontal. With referenceto anodes, it is to be understood that it is contemplated within thescope of this invention, that such anodes may be of the conventionalpro-baked type or of the conventional Soderberg self-baking type.

Due to the inclined or substantially vertical arrangement of theelectrodes, the floor space occupied by the cell is very considerablyreduced in relation to that which is at present required. Moreover, theelectrodes may be arranged to operate within a relatively confined bodyof molten flux or electrolyte and this may, in turn, be surrounded bysolidified fiux or electrolyte which may be retained in its desiredexternal shape by a simple Wall of steel or other suitable material, theconstructions of the cell being thereby considerably cheapened. Afurther great advantage of the cell construction embodying inclinedcathodes according to this invention is that the disposal of the noxiousor unpleasant fumes generated during the operation of the cell isconsiderably simplified, due to the much smaller area over which theyare evolved. It will also be appreciated that the cell may be designedin such a way that wasteful loss of heat is reduced to a minimum.

Yet a further advantage which flows from the inclined positioning of thecathode is that surging of the molten aluminum is very much less likelytooccur so that the spacing of the anode and cathode may besubstantially reduced compared with that adopted in cells are heretoforeknown and the dissipation of electrical energy energy in the electrolytecorrespondingly reduced. One further point of importance may be notedand this is that owing to the relatively high electrical conductivity ofthe cathode the voltage drop due to the passage of the operating currentis less than that experienced in cells of orthodox construction. Theefiect of sludge formation at the bottom of the cell, which is to causean undesirable voltage drop at the cathode in the existing horizontalcells, can readily be avoided in the operation of the new type of cellaccording to this invention. For example, I have constructed anelectrolytic reduction cell using a conventional electrolyte with atitanium carbide cathode and a carbon anode, both disposed substantiallyvertically. The cathode drop was found to be less than 0.2 volt,compared With the customary cathode drop of 0.5 to 0.7 volt encounteredin orthodox cells of comparable size, and the current eificiency wasalso found to be considerably higher than that obtainable in suchorthodox cells.

Current-conducting elements for use in electrolytic cells according tothis invention can be produced from the carbides and borides referred tohaving a reasonably low electrical resistivity, i.e., in the range offrom about 10 to microhms cm., a low solubility in molten aluminum, i.e.not more than about 0.04% in molten aluminum at about 970 C., a goodresistance to attack by the molten electrolyte employed in electrolyticcells for the production and refining of aluminum and a resistance tothermal shock such that the elements will withstand plunging into moltenaluminum at 750 C. at a temperature difierential not less than 200 C.without cracking. They are thus eminently suitable for use as leads fortaking part in the supply of electrolyzing current to a body of moltenalu- 7 minum in such a cell or a cathodes (or facings for cathodes) inelectrolytic reduction cells for the production of aluminum.

With regard to the carbides referred to above, titanium carbide ispreferred to zirconium carbide for the purposes in view, not onlybecause it is less expensive to produce but because it has a much higherresistance to oxidation than zironium carbide. When the latter isemployed, in fact, precautions must be taken to insure that it is neverexposed'to the action of air or oxygen or oxidizing conditions while ata high temperature, e.g. the operating temperature of the cell, forwhich reason a cathode or current-conducting element composed entirelyor consisting essentially of zirconium carbide should be protected, e.g.by protecting it with a sheath of oxidation resistant material beforeits temperature is raised to any substantial degree. In addition,zirconium carbide requires higher temperatures than does titaniumcarbide for the carrying out of the method of producing the coherentmass of the carbide having the requisite mechanical strength. For thesereasons, the following discussions with regard to carbides will beconcerned mainly with the use of titanium carbides but the several stepsand procedures detailed in this connection apply also in case of theother carbides, as well as in the case of the bordes, save where a noteis given of the necessary modification or where the necessity forprotecting the other carbides against oxidation will entailcorresponding precautionary measures.

The current-conducting elements, e.g. cathodes, current leads, etc., arepreferably prepared by providing the materials in the form of powders ofsuitable purity and particle size and then subjecting the material tocompacting and sintering operations by hot pressing which comprisessubjecting the powder, e.g. titanium carbide, to a continuously appliedpressure of from about 0.5 to 50 tons per square inch, e.g. 1 tonp.s.i., while raising its temperature to about 1600 to 2700 C., e.g.2000 C. Preferably the compacting and heating operations are carried outin a protective atmosphere, e.g. hydrogen or in a vacuum. It ispreferable to raise the temperature to the maximum value in a relativelyshort period of time, for example, in less than about 1 hour. Theoperation may be carried out in a graphite die having a cavity of theappropriate cross-sectional shape, the pressure preferably being appliedto the powder by plungers acting on opposite ends 4 of the column ofpowder and wherein the protective atmosphere is maintained around thedie during the heating and cooling periods. Although the above methodhas been found quite satisfactory in making current-conducting elements,it is contemplated within the scope of the instant invention that othermethods may be used, for example, fusing the materials in a hightemperature furnace and casting same to desired shape or the use of coldpressing and sintering techniques.

The cold-pressing method involves the cold pressing of the powders,followed by a sintering operation carried out at a temperature between1100 C. and 2200 C. either in vacuum or in a neutral atmosphere. Forexample, the material of the element, e.g. titanium carbide, may beprovided in particulate or powder form having a mean particle diameterof about 1 to 2 microns with which has been mixed a small portion of abinder, e.g. 1% of paraffin wax dissolved in benzene. The benzene isevaporated off on a water bath, or at a temperature sufficiently high tomelt the wax, prior to the compacting operation. The compacting step maybe effected by single pressures between male and female dies, either atordinary temperature or an elevated temperature, and wherein thepressure applied is in the range 0.5 to 50 tons per square inch, e.g. 3tons per square inch. Alternatively, the powdered mixture may beextruded into the desired shape. If the initial powder has asufficiently low content of free carbon, it may be directly compacted asabove and then pre-fired in vacuum to a temperature of 1100 C. (about1600 C. for zirconium carbide). It may then be worked by sawing, filing,or like shaping operations to produce an element of appropriate form,although it will be understood that the electrode or element willusually be finally shaped in the compacting operation. The electrode orcurrentconducting element is then fired in vacuum at an elevatedtemperature, e.g. where titanium carbide is the material of the elementthe firing temperature can be about 1600" C. (about 2200" C. forzirconium carbide) although this may be varied according to the densitydesired in the final product, to produce a robust, sintered elementhaving a porosity of the order of 20% by volume. Such an element is aself-bonded element, i.e. the titanium carbide particles adhere directlyto each other, and at such a relatively high porosity the pores areinterconnected and capillary paths exist within the element so that thelatter may be considered to be permeable.

Generally, the titanium carbide powder of commerce contains about oftitanium carbide and l to 2% of free carbon, the balance being titaniumoxide and titanium nitride in solid solution in the titanium carbide andcombined iron. If a powder of this character he treated as above setforth, that is, by cold pressing and sintering under pressures andtemperatures such that the element is permeable, the current-conductingelement obtained is not always suitable for the purposes intendedbecause the content of free-carbon is not necessarily reduced to a safelevel during a sintering operation carried out at a temperature of 1600"C. in vacuum. The use of a higher temperature either in vacuum or in afurnace in which an atmosphere of a neutral gas, such as hydrogen, ismaintained, results in improved products but some of these may stillhave a content of free carbon amounting to 0.6% by weight. It is foundthat a sintered titanium carbide compact having a porosity such that theelement produced is permeable and which contains more than about 0.5%free carbon shatters or disintegrates when wetted by molten aluminum,probably due to penetration of molten aluminum up the capillary pathsand internal reaction between the free carbon and aluminum to formaluminum carbide. Such an element would not be suitable for use as acathode or current lead exposed to molten aluminum in an electrolyticreduction cell for the production of aluminum. However, the difficultycan be overcome by incorporating into the titanium carbide powder beingused in the production of the element a suitable proportion of powderedcalcined alumina. Preferably, the alumina is added to the commercialtitanium carbide powder and the mixture then ball-milled for arelatively long period in the dry state until it is reduced to a meanparticle diameter of about 1 to 2 microns. The amount of alumina addeddepends to some extent upon the content of free carbon in the titaniumcarbide powder but is usually equivalent to about 2 to 3% of the weightof the titanium carbide employed.

The finely powdered mixture of titanium carbide and calcined alumina isthen moistured with a suitable binder, e.g. parafiin wax dissolved inbenzene, and the solvent driven off prior to compacting the mass underpressure as set forth above. The firing of the compact obtained iseffected in a furnace through which a stream of protective gas, e.g.hydrogen, is passed, the temperature preferably being higher than 1600C., for example, about 2200 C. (about 2700 C. for zirconium carbide).The finished product contains substantially no free carbon and theresidual aluminum therein, whether present as A1 0 or Al C is verysmall. As an example, a titanium carbide powder initially containingbetween 1% and 2% by weight of free carbon, when mixed with 2.5% byweight of calcined alumina and treated as above set forth, yielded anelement containing only 0.2% by weight of free carbon. Increasing theamount of A1 0 added to 3% resulted in an element containing 0.05% byweight of free carbon.

It is preferred that the content of free carbon in the finished elementshall be below 0.1% but somewhat higher percentages of free carbon canbe tolerated provided that the figure of 0.5% not be exceeded.Current-conducting elements thus prepared by the cold-pressed methodfrom substantially carbon-free titanium carbide will be wetted freely bymolten aluminum under cell operating conditions without any tendency toshatter or break up and without any sign of cracks developing.

Current-conducting elements made by the use of coldpressing techniques,at the temperature and pressure mentioned above, possess thedisadvantage of having a relatively high porosity, e.g. up to 20%, andof being permeable so that the elements can be penetrated by undesirablesubstances, e.g. flux or flux constituents. This disadvantage of fiuxpenetration can, however, be overcome by wetting and thoroughlyimpregnating the elements with aluminum at elevated temperatures, e.g.1100 to 1150 C., in vacuum. The aluminum completely wets all the exposedsurfaces of the elements and adheres thereto as a thin film. Thealuminum will penetrate into any pores of the elements and impregnatethem to a degree dependent upon the porosity thereof. The coating orimpregnation, for example, of a TiC compact, with aluminum improves theelectrical conductivity of the compact and it increases its resistanceto oxidation at high temperatures. However, as the electricalconductivity of TiC is adequate in itself the improvement in thisdirection, though useful, is not of great importance. On the other hand,the increase in the oxidation resistance obtained in the case of TiC isan advantage. Such porous impregnated elements should not, however, beemployed in positions where they are exposed to oxidizing atmospheres attemperatures above the melting point of aluminum as there appears to bea tendency for caustic attack of the elements possibly due to sodium, aproduct of the electrolysis, penetrating the elements via thealuminum-filled capillary passages and then oxidizing to caustic soda.

Permeable current-conducting elements formed from the cold-pressedmaterial are desirably pre-wetted and impregnated with aluminum beforebeing incorporated in an electrolytic cell as the wetting andimpregnation of the elements cannot be satisfactorily achieved at thenormal operating temperatures of the cell. A current-conducting elementmade by the cold-pressed method described above, i.e. by subjectingpowdered titanium carbide first to pressure and subsequently sinteringthe compact at a high temperature was found to have a porosity of theorder of 20% by volume, i.e. a density of 3.95 compared with thetheoretical density of 4.93, and a free carbon content of less than0.5%. It had an electrical resistivity of 54 microhm cm. and, whenimpregnated with aluminum, an electrical resistivity of 51 microhm cm.It had a transverse rupture modulus of 610 tons/ sq. in. and a thermalshock resistance to a temperature differential of from 200 C. to 300 C.Thermal shock resistance was measured by plunging a test bar (e.g. in.diameter and 4 in. long) into molten aluminum at 750 C. The test wasmade quantitive by heating the bar to various temperatures beforeimmersion and expressing the quality of the rod as the minimumtemperature difference between it and the molten aluminum which wouldcrack the bar.

The oxidation characteristics of the current-conducting elementimpregnated with aluminum expressed as weight increments in air at 1000C. was as follows:

was found to vary from 91% to 96% and the material had a titaniumnitride content of up to 5%. Iron was found to be present in amounts upto about 1%, but no particular effects are ascribed to it.

Current-conducting elements made by the hot-pressed method describedabove, i.e. by subjecting the powdered material simultaneously to heatand pressure, were found to be less porous than those produced by thecold-pressed method. For example, elements hot-pressed at 2000 C. undera pressure of 1 ton per square inch were found to have a porosity ofless than 10% and such elements are normally impermeable. Such elementshad a density of 4.4 as compared with the theoretical density of 4.93.Such elements made from hot-pressed titanium carbide can be Wetted withaluminum by vacuum treatment in the same way as the more porous materialreferred to above, but they cannot be impregnated with aluminum as thepores in the elements are not interconnected. However, in this case suchis not essential as wetting takes place automatically in the cell. Also,with hot-pressed elements the free carbon content of the material is notas critical and a dense material containing as much as 1% free carbonhas been found not to crack on wetting with molten aluminum. The reasonfor this is believed to be that with lower densities the free carbon isdispersed in the carbide as isolated particles of second phase and localreaction with molten aluminum occurs only on the surface of the element.

The electrical resistivity of the hot-pressed titanium carbide elementswas found to vary With the composition of the material. Thus withelements prepared from mineral rutile the electrical resistivity variedsubstantially linearly from 84 microhm cm. at by weight of titaniumcarbide to 63 microhm cm. at 96% by weight of titanium carbide. Thevalues were, however, markedly lower when elements were prepared frompigment titanium oxide. This is ascribed partially to the lowerzirconium and vanadium contents of the pigment oxide and also to a loweroxygen content of the material. However, a resistivity of 68 microhrncm. and .a linear temperature coefficient up to 1000 C. of about 0.0008/C. is considered to be reasonable and may be readily attained.

The hot-pressed titanium carbide elements were found to have atransverse rupture modulus of 15-20 tons/sq. in., a thermal conductivityof about 0.07 c.g.s. and a thermal expansion over the temperature rangeof 20 C.- 400 C. of 7.2 l0 cm./cm./ C. and over the temperature range of20 C.1000 C. of 81x10" cm./cm./ C. The elements were further found tohave a thermal shock resistance to a temperature differential in excessof 300 C. V

The hot-pressed titanium carbide elements were found to contain upwardsof 90% by weight of titanium carbide, (usually not more than 96% byweight), up to 5% of titanium nitride and a free carbon content and ironcontent of up to 1% by weight each. The presence of the nitride, freecarbon and iron had no apparent deleterious effect on the hot-pressedimpermeable titanium carbide elements.

Although the carbides have a good resistance to aerial oxidationrelative to carbon suitable protection should desirably be providedwhere these materials are used as current-conducting elements inelectrolytic cells in positions in which they are exposed to suchoxidation. They are advantageously provided with a sheath as willhereinafter be described. Titanium carbide is subject to a penetratingform of oxidation at a critical temperature of about 450 C. and thistype of attack at this lower temperature is thought to be due to theformation of a dis integrated and non-protective metal oxide. Theoxidation resistance of this material, however, increases attemperatures above 450 C. to a maximum value at about 700 C.-750 C. Atstill higher temperatures its oxidation resistance decreasesprogressively.

With regard to the effect of impurity content in the use of titaniumcarbide it has been found that oxygen tends to adversely affect thesolubility of titanium carbide in molten aluminum in the temperaturerange normally found in operation of the cell, e.g. 950 C. to 1050 C.for a reduction cell. The oxygen present is in solution in the titaniumcarbide and probably is present in the form of titanium monoxide. It hasbeen found that there is a strong tendency for the carbide todisintegrate where oxygen is present above 1% by weight. A reasonablygood commercial product contains about 0.5% by weight of oxygen and thishas been found to have a solubility in molten aluminum at 970 C. ofabout 0.02% titanium. Preferably the oxygen content in the carbideshould be maintained in an amount less than 0.5%.

It will be appreciated from the foregoing that a currentconductingelement of titanium carbide should have an oxygen content of less thanabout 1% by weight. Such an element when permeable, e.g. when made bythe coldpressing method and having a porosity of about 20% by volume,should have a free carbon content which is not greater than about 0.5%and it should desirably be prewetted and impregnated with aluminum priorto its incorporation in an electrolytic cell. An element which issubstantially impermeable, i.e. having pores which are notinterconnected, e.g. one made by the hot-pressing method and having aporosity not greater than about by volume, does not have the limitationas to its free carbon content and need not be pre-Wetted with aluminum.It may, however, advantageously be pre-wetted as such prewetting is auseful indication of the satisfactory nature of the material. If theelement stands up to the wetting step Without cracking, it is reasonablycertain that the element will prove satisfactory in service and notdisintegrate in the cell. Finally, the pre-wetting of the element withaluminum facilitates the making of the preferred form of connectionbetween the element and the external current-supply bus-bars feeding thecell. This preferred form of connection is a bar of aluminum which iscast onto the element at its one end to be in intimate electricalcontact with the carbide. The product obtained by any of the methods setforth above is a shaped current-conducting element, e.g. a cathode,which can be effectively wetted with aluminum. When an element has beenso wetted, a bar of pure aluminum to serve as an electrical conductormay readily be fused directly thereto. Consequently, when the cathode isin service, it may be connected to a source of electrolyzing current bya bus-bar directly fused or cast onto one end of the cathode which isnot exposed to the interior of the cell, that portion of the cathodeexposed to the interior of the cell being constituted by the refractorymaterial coated with molten aluminum. it will be appreciated thatelectrical losses are low by reason of this construction.

When the cathode is employed at an appreciable inclination to thehorizontal, the aluminum which is continuously deposited on the surfaceof the cathode while the cell is in operation runs down the latter tocollect in a pool in the lower part of the cell, this pool preferablybeing in contact with the cathode. It should be noted, however, that thecathode remains completely wet-ted by molten aluminum which adherestenaciously thereto. A cell having an inclined cathode constructed inaccordance with the invention may have anodes of carbon constructed andfed in any suitable manner and will operate on a voltage which isconsiderably less than that required in cells of comparable sizeemploying cathodes of the orthodox type. Moreover, the currentelficiency is higher than it hasbcen possible to achieve in practicewith cells of the usual construction.

With further reference to the refractory materials referred to abovespecial reference should be made to titanium boride (TiB and zirconiumboride (ZrB which have similar properties to each other, theseproperties being superior to those of the carbides for the purposes inView and not hitherto known. Titanium boride is readily wetted by'moltenaluminum under cell operating conditions and has a much lower electricalresistivity than titanium carbide (1040 microhm cm. measured at 20 C.);has more resistance to oxidation than titanium carbide over thetemperature range 300-750 C. (this being very marked at temperatures ofabout 450 C.) and at temperatures above about 850 C. due to the normalformation of a glassy oxidation phase; and has a solubility in moltenaluminum at temperatures of .the order of 970 C. which is only aboutone-tenth of that of titanium carbide. It will thus-be seen that theborido of titanium is preferred to the carbide for the purposes in view.

However, all the materials referred to have electrical resistivitieswhich are sufficiently low to make them suitable for use in theproduction of current-conducting elements for employment in electrolyticcells for the production of aluminum.

The lower electrical resistivity of the borides (particularly titaniumboride) is of practical importance as it enables an economy to be madein the cross-section of the current-conducting elements and/or in thenumber of such elements required and this helps to off-set the greatercost of these materials.

In addition to the characteristics mentioned above, the borides, andparticularly titanium boride (TiB and zirconium boride (ZrB have othercharacteristics which render them particularly suitable for the purposesin view. The free carbon content of titanium boride has not been foundto be in any way limiting. Titanium boride asnormally manufactured doesnot have a high oxygen content but titanium boride is not sensitive tothis oxygen content at least to the same degree as'titanium carbide.Thus, impermeable current-conducting elements formed from hot-pressedtitanium boride having an oxygen content of 1.4% have operatedsatisfactorily in reduction cell tests. The purity of the materialemployed does not appear to be critical. For example, tests have beenmade with titanium boride materials having quite high impurity contents,e.g. up to 1% by weight each of free carbon and nitrogen, excess boron,carbon and iron in combined forms. Current-conducting elements composedof these materials, hot-pressed, revealed no apparent deleterious effectas regards cracking or penetration and dissolved very slowly anduniformly in molten aluminum. The purity of the material does, however,have an effect upon its electrical resistivity and can vary it by afactor of about 4 at room temperature, i.e. from 10 to 40 microhms cm.so that it is desirable to control the purity from this point of wow.

As mentioned above, the solubility characteristics of the borides isvery much more favorable than the carbides. Thus in the reaction TiB Ti(dissolved) +2B (dissolved) the stoichiometric solution of the compoundat about 970 leads to titanium and boron contamination in the aluminumof about 0.004% and 0.0015% respectively, i.e. about one-tenth of thetitanium content observed with the carbide. This advantage of theborides, particularly titanium boride and zirconium boride, was quiteunexpected and could not have been predicted.

A further advantage of the borides is that, for example, in theequilibrium solution of TiB in molten aluminum both titanium and boronare below their saturated solubilities and throughout a wide temperaturerange the simple solubility product law (Ti) (B -=constant; defines theconditions for the solution or precipitation of titanium boride. Thedissolution of the elements can thus be hindered by the addition oftitanium, or very much more effectively of boron, to the molten metal inthe cell.

A certain amount of titanium is normally present in the metal from othersources and this prolongs the life of the current-conducting elements tosome extent. The useful life of current-conducting elements composed ofthe borides is, however, quite long enough for the purposes in view sothat normally it is not necessary to increase it by the addition oftitanium or boron to the molten aluminum. However, if it is desired toincrease the life of the elements composed of one or more of theborides, then boron may be added to the molten aluminum in theproportion of from 0.001% to 0.003% by weight or alternatively the metalof the boride, e.g. titanium, may be added in the proportion of from0.001% to 0.005% by weight. It will be appreciated that, as a furtheralternative, a small proportion of both boron and the metal of theboride may be added to the molten aluminum.

As an example, tests have been carried out with current-conductingelements produced from hot-pressed titanium boride. The density of theseelements was found to vary from 4.1 to 4.3 (theoretical density 4.52)and the porosity of the elements was always less than 10%. The elementshad a thermal expansion over the temperature range of 20 C.400 C. of 6.910 cm./cm./ C. and over the range of 20 C.l000 C. of 8.0 l cn1./ cm./ C.The electrical resistivity of these elements varied from to 40 microhmscm. at C. Elements having a resistivity of between 10 and 20 microhmscm. at room temperature were shown to have a linearresistivity-temperature relationship and a coefficient of 0.003/ C. Theelements were shown to have a transverse rupture modulus of 1520tons/sq. in. The elements had a thermal shock resistance to atemperature dilferential of 350 C.

The elements had an oxidation resistance which was better than thoseproduced from titanium carbide. This is most marked at 450 C. at whichtemperature the titanium boride elements showed a depth of penetrationof 8 microns after 120 hours whereas the titanium carbide elementsshowed a depth of penetration of 88 microns.

I have determined the approximate value of the solubility product oftitanium and boron in solution in molten aluminum at differenttemperatures and the following table sets out some examples of thisproduct and the corresponding titanium concentration Which is thereforeobserved when titanium boride (Til3 dissolves in the aluminum in itscorrect stoichiometric proportions.

Temperature, (Ti) X (B Ti for stoichio- C metric solution (percent) 1100b l0- 0. 008 970 1X10- 0. 004 850 2X10- 0. 0026 Titanium boridematerials may in practice contain a small excess of boron whichdissolves preferentially in the molten aluminum and so reduces thesolubility of the titanium in accordance with the above relationship.This effect, however, is usually transient.

As has been mentioned above, the purity of the boride material employedhad no appreciable eiiect on the penetration of the elements. Mentionshould, however, be made of the fact that materials having 10% by weightof boron carbide deliberately added thereto were found to be subject topenetration effects.

Tests on current-conducting elements composed of hotpressed zirconiumboride showed this material to be comparable with titanium boride.

As a further feature of the invention it has been found that specialadvantages can be secured by the use of mixtures of titanium carbide andtitanium boride, which would not be anticipated from a study of theirproperties determined separately.

The mixture may contain various proportions of the two ingredients,depending upon the properties required in the final product, and in somecases, the current-conducting element may be composed of a mixture thecomposition of which progressively changes along the length of theelement.

The most surprising effect obtained by admixing titanium boride withtitanium carbide, when preparing the current-conducting elements, is thedecrease in the solubility of the titanium carbide in aluminum at hightemperature (eg. at 970 C.) as shown in FIGURE 1 of the drawings. Quitesmall percentage additions of the boride appreciably suppress thesolubility of the carbide. As will be seen from FIGURE 1, the solubilityof a titanium carbide sample decreased to approximately one-third by theaddition of only 10% of titanium boride and that it then remainedsubstantially constant up to at least 70% of the boride.

Since titanium boride is an expensive material compared with titaniumcarbide, it is of great value to be able to secure substantialimprovements in solubility by the incorporation of relatively smallportions of the horide in the cheaper and more readily availablecarbide. Generally, additions of the order of 5 to 25% by weight areadequate, the preferred amount being in the range from about 10 to 20%by weight. Additions of less than 10% will produce beneficial results,and it will then be a question of balancing the increased cost of theboridecontaining material against the longer life of thecurrentconducting elements which would be obtained under cell conditionsalthough it is preferred to add not less than 5%.

As far as other properties are concerned, the titanium carbide-titaniumboride mixtures appear to be intermediate between the carbide andboride. Thus the resistance of the mixture to oxidation in air at hightemperatures is better than that of titanium carbide alone. The electrical resistivity does not deviate very markedly from a linearrelationship with composition and decreases from a value between andmicrohms cm. for titanium carbide to approximately 10 microhms cm. fortitanium boride. There is thus an additional advantage to be gained fromthe use of the mixtures, which offsets the higher cost of the boride, inthat an element of smaller cross-section can be used to carry a givencurrent.

The carbide-boride mixtures may be prepared by several methods ashereinbefore discussed. It should be noted that it is now possible toproduce a current-conducting element for use in either reduction orthreelayer cells which is complex in composition. Thus it is possible,for example, to produce a bar element which contains 20% of titaniumboride and of titanium carbide over only a portion of its length, theremainder of it consisting essentially of titanium carbide only. Interinediately between the two links of different composition, theproportions of the ingredients may, if so desired, be changedprogressively so that a graded joint is obtained. By this procedure, therisk of cracking at the juncture between the two links of differentcompositions is minimized. Also, it is feasible to producecurrent-conducting elements which consist of a mixture of carbide andboride where they protrude into the molten cathode pool of a cell, andof carbide only where they are buried in the wall or bottom of the cellitself.

It is further feasible to produce current-conducting elements whichconsist of the borides only where they protrude into the molten metal,the remainder of the element consisting of the carbide only. This ispossible as the coefiicient of expansion of the boride is very similarto that for the carbide.

In order that practical methods of utilizing currentconducting elementsaccording to this invention may be more clearly understood, referencewill now be made to the accompanying drawings which illustrate, somewhatdiagrammatically, several examples of electrolytic reduction cells inwhich the cathodes are solid conducting elements disposed at anappreciable inclination to the hori zontal or are pools of moltenaluminum electrically connected to a source of electrolyzing currentthrough one 15 or more current-conducting elements according to thisinvention. Also, the drawings illustrate examples of the invention asembodied in three-layer purification cells. 7

Referring first to the example illustrated in FIGS. 2 and 3, it will beseen that the cell is of rectangular shape both in plan view and intransverse section and comprises an outer wall 1 of a refractoryinsulating material, such as magnesite, and an inner wall or lining 2 ofcarbon. Vertical partition walls 3, also of carbon, are provided toextend inwards from the longitudinal side walls of the carbon lining,each partition wall being in contact at its outer and bottom faces withthe respective surfaces of the carbon lining 2 but terminating short ofthe longitudinal center line of the cell. The inner face of each wall 3is inclined upwards and outwards from its lower end at a relativelysteep inclination to the horizontal and the upper face of the wall islocated at a level somewhat below that of the upper surface of themolten electrolyte 4 which fills the cell when the latter is inoperation. The partition walls 3 are spaced apart along the length ofthe cell and are arranged in opposed pairs, one wall of each pairserving to support the respective one of a pair of cathodes 5 adapted tocooperate with an anode 6 supported between them.

Each cathode 5 is composed of a rectangular plate of sintered carbideand/ or boride, which is advantageously produced in the manner set forthin the foregoing description. This cathode plate rests by the centralportion of its outer face against the inner face of the correspondingpartition wall 3 and its lower edge is disposed in contact with therespective side wall of a channel 7 formed longitudinally of the innerface of the base of the carbon lining 2 and of a width to extend fromone Wall 3 of a pair to the other. It will be seen that in thearrangement described there are located at intervals along the length ofthe cell pairs of opposed cathode plates 5 arranged in a V formationwith their lower edges spaced apart by the width of the channel 7. Theupper edges of the cathode plates, as shown in FIG. 2, project somewhatabove the upper faces of the partition Walls 3 to terminate short of theupper surface of the molten electrolyte filling 4.

The anode 6 is constructed from carbon and is of rectangular shape inany horizontal section but has its lower or operative end portion formedof wedge shape so that its inclined rectangular faces 6a are disposedsubstantially parallel with the inner faces of the respective cathodeplates 5. The anode 6 is supported by a hanger 8 (FIG. 2) of aluminum,iron or copper electrically connected to a bus-bar (not shown) toconnect the anode to the positive pole of the source of supply ofelectrolyzing current. The upper portion of the anode extends above thelevel of the molten electrolyte filling 4 and through the crust 4a ofsolidified or frozen electrolyte overlying the same. As the anode isconsumed during the operation of the cell it is progressively feddownwards in the wellknown manner. The location of the inclined faces ofthe anode is such that the desired small inter-electrode distance willalways be insured.

In the angle between one face of each partition wall 3 and the adjacentlongitudinal face of the carbon lining 2 of the cell there is formed inthe base of the lining a shallow depression 9 within which is disposedthe inner end portion of a bar 10 formed from one or more of thecarbides and borides set forth in the above description. The bar 10 isproduced in the same manner as the cathode plates as set forth above.This bar constitutes a current lead and extends horizontally outwardsthrough the vertical wall of the carbon lining 2 to be electricallyconnected to an aluminum bus-bar 11 which has its inner end embedded inthe insulating wall 1. This bus-bar 11 may be connected to the currentlead in the manner described in the foregoing in relation to theattachment of a current conductor to a carbide cathode, that is to say,

the inner end of the bus-bar may be cast onto the current 16 lead whichhas previously been Wetted with molten aluminum. The bus-bars 11 areconnected to the negative pole of the source of supply of theelectrolyzing current.

With reference to the receptacle portion 1, 2 of the above describedcell, it may be said that the receptacle generally defines a chamberhaving an upper zone adapted to contain a body of solidified flux, alower zone adapted to receive a pool of molten aluminum and anotherelectrode body in the form of a current-conducting element and anintermediate zone adapted to contain a body or charge of moltenelectrolyte or flux.

In preparing the cell for production, the electrolysis may be started byone of the several procedures known in the industry. For example, theanodes 6 may be lowered until the are in contact with cathode plates 5and the base of the carbon box 2. By passing current through theelectrodes and box the assembly may be heated to an appropriatetemperature, say 700 C. The anodes may then be raised out of contactwith cathode plates 5 and the box 2 while molten aluminum is poured intothe cell to form a pool 12 covering the fioor thereof, followed bymolten electrolyte, e.g. molten cryolite containing dissolved alumina.When the cell is filled to the desired level the full electrolyzingcurrent is supplied to the electrodes and the cell brought into fullproduction.

When the cell is in full operation, the major pro portion of theelectrolyte is in the molten state, although there will form a crust 4aof solid or frozen electrolyte bridging the gap between the carbonlining 2 of the cell and the respective anodes 6 and also extending downthe walls of the lining as indicated in FIG. 2. Aluminum is now beingdeposited on the whole of the exposed surfaces of the cathode plates 5,it being in the molten state, and runs down these surfaces into the pool12 of molten metal extending over the base of the carbon lining 2, whichpool also fills the channel 7. The pool constitutes the electricalconnection between the current leads 10 and the cathode plates 5,substantially all the electrolyzing current being conducted by thealuminum and little or none by the carbon base. The molten aluminum maybe tapped off from this pool from time to time as required.

In the arrangement shown in FIGS. 4-6 the construction of the cell issubstantially the same as that shown in FIGS. 2 and 3 so far as theinsulating outer wall 1, carbon lining 2, partition walls 3, cathodeplates 5, anodes 6 and channel 7 are concerned.

In this case, however, the current leads 10 shown in FIGS. 2 and 3 areomitted and the connections between the cathode plates 5 and thenegative pole of the current supply source comprise extensions 5a formedintegrally on the upper edges of the cathode plates to project upwardsthrough the solidified crust 4a of electrolyte. Each of these extensions5a is connected to an aluminum busbar 11, for example, by having theadjacent end of this bar cast thereon or brazed thereto. In this casethe shallow depressions 9 provided in the construction according toFIGS. 2 and 3 are not required. Desirably, although not absolutelynecessary, in order to enhance the resistance of the extensions 5aagainst oxidation and corrosion where they pass through and emerge fromthe crust 4a they are surrounded over the relevant part of their lengthby a sleeve 5b consisting of a suitable refractory material, forexample, that known as Refrax which is silicon carbide bonded withsilicon nitride. Alternatively, the material may be hot pressed siliconcarbide.

The method of operation of this cell is substantially the same as thatshown in FIGS. 2 and 3 but, in this case, the supply of current to theplates 5 is effected through the extensions 51:.

FIGS. 4 to 6 additionally illustrate a hood 13 seating by its lower edgeon the upper edge on the insulating wall 1 and having its interiorconnected by a duct 14 to a fume extraction plant (not shown). Suitableapertures are provided in the hood to permit the passage of the bus-barswhich serve to conduct current to the cathode plates and anodes,respectively. It will be appreciated that a similar hood may be providedin the arrangement shown in FIGS. 2 and 3.

In both cells illustrated, aluminum is deposited electrolytically uponthe inclined faces of the cathode plates and runs down these faces tocollect in a pool 12 on the bottom of the cell. If the plates 5 were notwetted with aluminum before being built into the cell, the firstquantity of aluminum deposited thereon serves to effect adequate wettingof their faces but once this has occurred, the aluminum subsequentlydeposited trickles to the pool as mentioned above. In some cases it maybe preferred to employ cathode plates which have been wetted with moltenaluminum, by the method set forth above, before they are introduced intothe cell. The aluminum produced by electrolysis will then run down thecathode plates from the start.

It should be noted that the cell may be operated in such a manner thatthere is a thicker crust 4a of solidified electrolyte surrounding theinner molten portion of the electrolyte than is shown in the drawings,in which case the whole body of electrolyte may be contained in a simplebox of steel or other suitable material.

It will be appreciated that the cathodes 5 of the arrangementsillustrated in FIGS. 2 to 6 would be relatively large and they would beexpensive to manufacture. This expense may be reduced by making thecathodes 5 of carbon coated with one or more of the carbides and boridesreferred to, preferably titanium boride.

FIGURES 7 to 10 illustrate modifications of orthodox electrolyticreduction cells according to this invention.

With reference to FIG. 7, the cell shown has a base or plinth 21 of arefractory insulating material, such as magnesite, upon which issupported a shallow box structure 22 composed of carbon. This structureis supported and contained by a surrounding Wall 23 of steel.

Along the longitudinal edges of the upper surface of the bottom of thebox 22 there are formed two shallow channels 24 into which project, atintervals along the length of the cell, current-conducting elements 25of bar form composed of at least one of the carbides and boridesmentioned above. In this arrangement, each bar 25 extends horizontallythrough the wall of the box 22 to project into the adjacent longitudinalchannel 24, its outer end being attached to a bus-bar 26 of purealuminum which is connected to the negative pole of the source of supplyof the electrolyzing current. The end of the bus-bar 26 may be castaround the adjacent end of the bar 25.

The anode 27 is of carbon and is connected by suitable means (not shown)to the positive pole of the source of supply of electrolyzing current.The bus-bars 26 are connected, as at 26a on the left-hand side of FIG.7, to main bus-bars 28 which extend along the sides of the cell.

The cell may be brought into operation by one of the several proceduresknown in the industry. For example, the anode 27 and the box 22 may beheated to the operating temperature by lowering the anode onto suitablecarbon blocks placed on the base of the box and passing electric currentthrough them. After the blocks have been removed, electrolysis can bestarted by pouring in molten aluminum to form a pool 29 covering thebars 255, adding molten electrolyte 30 containing dissolved alumina andimmediately passing the full electrolyzing current through the cell. Thepool of molten metal effectively constitutes the cathode for the cell.When the latter is in full operation, the major part of the electrolyteis maintained in the molten state, as shown at 30, and is covered by acrust 31 of solid or frozen electrolyte.

With reference to the receptacle (portions 21, 22, 23) of cells of theabove described type, it may be said that the receptacle generallydefines a chamber having a 13 lower zone (horizontal) adapted to receivea body of molten aluminum, an intermediate zone (horizontal) adapted toreceive a body or charge of molten electrolyte or flux, and an upperzone (horizontal) adapted to contain therein a layer of solidifiedelectrolyte or flux. As is apparent, the anode is disposed within thezone of electrolyte, both solidified and molten. Alternatively, it maybe said that the receptacle defines an upper zone (horizontal) adaptedto receive a first electrode body, a lower zone (horizontal) adapted toreceive a second electrode body, and an intermediate zone adapted tocontain a body or charge of molten electrolyte or flux.

FIG. 8 illustrates another arrangement of the currentconducting elements25 where each of them (only one is shown in the figure) is introducedinto the cell from above, passing through the crust 31 of solidifiedelectrolyte which extends over the body 30 of molten electrolyte andbridges the gap between the anode 27 and the margins of the cell. Theelement 25 may be provided with a sheath 32 similar to the sheath 5bdescribed with reference to FIGS. 4, 5 and 6. The current-conductingelement 25 extends down the inner face of the appropriate side wall ofthe cell to terminate at its lower end in close proximity to the base ofthe cell. When the latter is in operation a pool 29 of molten aluminumcollects on the base and submerges the lower end of the element 25.

in the alternative arrangement shown in FIG. 9, the current-conductingelements 25 are disposed vertically and inserted through the base of thecarbon box 22 of the cell. The upper ends of the elements 25 project fora short distance above the inner surface of the carbon base of the cellinto the lower zone of the chamber defined by box 22 and effectivelyestablish electrical connection between the pool 29 of molten metalwhich collects in the lower zone on this base and the negative busbars26b which are shown as electrically connected to a main bus-bar 2st.extending beneath the cell.

FIGS. 10 and 11 illustrate a further modification of an orthodoxreduction cell by incorporating therein current-conducting elementsaccording to this invention. As is usual, the base of the box 22 iscomposed of blocks of graphitic material in which there are embeddediron bars 33 serving to connect the blocks electrically to negativebus-bars (not shown) located externally of the cell. Such cells sufferfrom the disadvantage that a high resistance electrical contact isnormally present between the molten aluminum 29 and the box 22, due tothe fact that the metal does not Wet carbon and a poorly conductingsludge settles on the latter during the operation of the cell.

In order to overcome this disadvantage, current-conducting elements 25a,in the form of rods of cylindrical or other shape composed of one ormore of the carbides and borides referred to above, are inserted inbores formed in the blocks of the cell bottom, these rod elements beingof a length somewhat greater than the depth of the bores so that theirupper ends project into the molten metal 29 and provide low-resistancepaths for the electrolyzing current which short-circuit the sludgelayer. The bores are preferably formed so that they are uniformlydistributed over the bottom of the cell at locations substantiallyregistering with those of the iron bars 33 but terminate short of theseso that there will be an intervening solid portion of the carbon of theblock which will obviate leakage of the cell contents. The elements 25aare preferably bonded in position by means of a thin layer of pitchwhich is converted into a solid carbonaceous binder at the temperatureof operation of the cell. Instead of the pro-baked anodes indicated inFIGS. 10 and 11 a self-baking anode may, of course, be used and in thiscase it is continuously renewed from above in the well-known manner bysupplying a carbonaceous material to its upper end. The same applies forthe anodes of FIGS. 7 to 9.

A test was carried out on an orthodox reduction cell modified in themanner illustrated in FIGS. 10 and 11 and a careful comparison made withan entirely orthodox or control cell run in series with it and operatedunder identical conditions. The modified cell had inserted verticallyinto the carbon fioor an appropriate number (24) of hot pressed titaniumcarbide bars 2 inches in diameter and 7 inches long, uniformlydistributed over the floor and arranged so that 1 4 inches of the barsprojected above the carbon cathode. The control cell was entirelysimilar except for the provision of these carbide bars.

The two cells were observed carefully over a period of approximately 3months, the average current through them during this time being 16,000amperes. The electrolyte used was conventional sodium cryolitecontaining excess aluminum fluoride (ME) and a small amount of calciumfluoride (CaF the latter being controlled in the range 610% throughoutthe experiment. With regard to the aluminum fluoride content the fluxcomposition was controlled so that the excess of that required to formcryolite (3NaF.AlF over the 3 month period averaged 6.8% for the carbidemodified cell and 6.5% for the control or orthodox cell. The cells wereoperated in the well-known manner in which aluminum oxide (A1 is fedinto the electrolyte at regular intervals; the percentage of aluminum inthe electrolyte was thus about 5% when this addition had been freshlymade and fell slowly to 0.5-1.0% when an anode effect occurred, i.e. thevoltage across the cell increased to a relatively high value, and afurther addition was required.

Regular measurements were made of the temperature of the fiux and alsoof the aluminum metal pool at the bottom of the cell; the average valuesover the 3 month period were as follows below:

The average cathode voltage drop in the control cell over the testperiod was 0.58 volt, the individual readings varying between 0.45 and0.81 volt. The corresponding average cathode voltage drop on the carbidemodified cell was 0.38 volt and the individual values varied between0.30 and 0.46 volt. The control or orthodox cell had a currentefliciency over the test period of 89.8%, whereas the modified cell hada current efiiciency of 90.8% over the same period. The metal producedby the modified cell had a titanium content approximately 0.02% higher.than that for the control cell.

A modified cell constructed in accordance with the embodiments shown inFIGS. and 11, in which the elements a consisted of hot-pressed titaniumboride bars 2 inches in diameter and 9 to 9 /2 inches in length, wasalso compared with an unmodified orthodox or control cell run in serieswith it. These cells were of a larger type than those given in theexample above, and the series current was about 40,000 amperes. Thenumber of titanium boride elements inserted in the floor of the testcell was 27, i.e. due to the lower electrical resistivity of the borideas compared with the carbide each boride bar carries twice as muchcurrent as a corresponding carbide bar for the same voltage drop.

The cells were operated for a test period of five months under similarconditions to those already described with reference to the cellmodified by the titanium carbide leads with the sodium cryoliteelectrolyte containing approximately 8% calcium fluoride (CaF and withan excess aluminum fluoride content of about 3-5%. The average cathodevoltage drop of the modified cell over the test period was 0.2 volt lessthan that of the control 'cell and the current efiiciency was also about2.0% higher. The average titanium and boron contents in the metal 20produced by the. two cells .was as follows: Titanium boride modifiedcell 0.007% titanium, 0.001% boron. Control cell 0.0045 titanium,0.0002% boron.

Current-conducting elements 25a consisting essentially of hot-pressedtitanium boride have been tested continuously over a period of fivemonths in a reduction cell such as is illustrated in FIGS. 10 and 11.The elements 25a were in the form of cylindrical bars having a diameterof 2 inches and a length of 4 inches and had a porosity of about 6% byvolume. The material when analyzed was found to have the followingcomposition, the percentages being by weight:

Percent Soluble boron 29.53 Free carbon 0.34 Combined carbon 0.43Nitrogen 0.68 Oxygen 1.37 Iron 0.88 Insoluble boron 1.92

From this analysis it is estimated that the proportion of titaniumboride (TiB present was about The initial solubility of the elements wasfound to be .002% Ti and 0.002% boron.

These elements when removed at the end of the five months test periodwere found to be in a sound condition and the end thereof exposed to themolten aluminum had been uniformly reduced in diameter by about 0.4inch.

FIGS. 12 and 13 illustrate a still further arrangement of thecurrent-conducting elements wherein each of the elements 34 isintroduced into the cell from above passing through the crust 31 ofsolidified electrolyte which extends over the body 30 of moltenelectrolyte through the body 30 of molten electrolyte and into the pool29 of molten aluminum. The container or box 22a is composed of anysuitable refractory material which is resistant to attack by moltenaluminum, electrolyte and oxidizing atmospheres. It is not essential forthis material to be electrically conducting and it may thus, forexample, be composed of the material known as Refrax, i.e. siliconcarbide bonded with silicon nitride, or hot-pressed silicon carbide. Twoparallel spaced rows of carbon anodes 27 extend substantially verticallythrough the crust 31 into the body 30 of molten electrolyte. Theelements 34 are disposed in a row between and parallel to the anodes 27with their lower ends in close proximity to, but spaced from, the floorof the container 22a. The elements 34 are conveniently in the form ofcylindrical bars and there may be, for example, three elements disposedbetween each opposed pair of anodes 27. A sleeve 35 may be provided onthat part of each element exposed, this sleeve being a suitablerefractory material, e.g. the material known as Refrax or hot-pressedsilicon carbide. The anodes 27 are suspended by hangers 36 through whichthey are connected to parallel bus-bars 37 and 38 (FIG. 13) adapted tobe connected to the positive pole of a source of electrolyzing current.The bus-bar 37 is common to one row of anodes 27 and the bus-bar 38 iscommon to the other row of anodes 27. The elements 34 are suspended byhangers 39 which are conveniently of aluminum cast around the upper endsof the elements 34 and the hangers are connected to a common bus-bar 40which is parallel to the bus-bars 37 and 38 and is adapted to beconnected to the negative pole of the current source.

The bus-bars 37, 38 and 40 are connected to the respective poles of thecurrent source in such manner that the current flow along the bus-bars37 and 38 is in one direction and the current flow along the bus-bar 40is in the opposite direction. This can be achieved by connecting, forexample, the right-hand ends of the bus-bars 3'7, 33 and 4-0 as seen inFIG. 13 to the respective poles of the source of electrolyzing current.The advantage of this is that it reduces to a minimum the effects of themagnetic fields produced by the currents due to the fact that the fieldproduced by the current in the bus-bar 40 tends to neutralize the fieldsproduced by the currents in the bus-bars 37 and 38. This is ofimportance as, without this arrangement, the magnetic fields would havean undesirable agitating and stirring effect upon the contents of thecell. By adjusting the relative heights of the bus-bars 37, 38 and 4twtih respect to the cell the magnetic fields may be arrangedsubstantially to neutralize each other, at least so far as the cellcontents are concerned.

An advantage of the arrangement described with reference to FIGS. 12 and13 is that the elements 34 are not built into the cell and may bereadily replaced as required.

Reduction cells are usually operated with a cryolitecalcium fluorideelectrolyte and at a temperature of 950 to 970 C. It has been proposedto operate reduction cells employing conventional anodes and cathodes ofcarbon and graphite with an electrolyte bath containing sodium chlorideas a constituent. The advantages occurring from the use of a sodiumchloride type of electrolyte stem from the fact that the cell operatesat a lower temperature than those employing the conventionalelectrolyte, e.g. at 920 C. On the other hand, the solubility of aluminain the sodium chloride type of electrolyte is lower, but this ispartially off-set by a lowering of the alumina content at which theanode effect occurs, i.e. at about 0.4% by weight of alumina comparedwith 1.5 to 2% in the normal bath. The lower solubility of the aluminaleads to difficulties in conventional reduction cells, however, becauseit is difficult to avoid overfeeding the alumina to the cell and theexcess settles on the floor of the cell in the form of a sludge. As, inconventional reduction cells, the electrical connection to the moltenpool of aluminum is through the floor of the cell this impairs theefiicient operation of the cell.

This disadvantage of the overfeeding due to the reduced solubility ofthe alumina in the NaCl-type electrolyte can easily be overcome bypractice of the instant invention wherein the current-conductingelements may be arranged to project into the molten metal above thelevel of the sludge. They may extend through a side wall of the cell,through the cathode bottom thereof, or downwardly through theelectrolyte thereby maintaining good electrical connection with themetal at all times.

Accordingly, it will thus be seen that the cell structure of theinvention permits the use of sodium chloride containing electrolytes andbecause the dissolution of the current-conducting element in thealuminum produced in the cell is a function of the temperature, the useof sodium chloride type of electrolyte makes it possible to prolong thelife of the element by reason of the lower temperature at which the cellcan then be operated. The reduction of the operating temperature byabout 40 C., which it is possible to achieve, has a material effect uponthe life of the element.

As an example only, the fused salt bath employed may contain sodiumcryolite and sodium chloride, the latter constituting between about and30% by weight of the bath.

Reduction cells such as are illustrated in FIGS. 2 to 13 mayadvantageously utilize a bath containing sodium chloride as aconstituent.

FIGS. 14 and show the application of the invention to three-layerpurification cells. In both figures a current-conducting element 45,which is advantageously composed of one or more of the carbides andborides referred to, extends horizontally through the insulating(magnesite) wall 46 of the cell to project by its inner end into adepression 47 formed in the base of the cell so that, when the cell isin operation, it will be submerged in the body 48 of molten aluminumalloy which constitutes the bottom layer. The outer end of the element45 is connected to an aluminum bus-bar 49 (which may be cast thereon)leading to the positive pole of the source of sup ply of theelectrolyzing current.

In FIG. 14 current-conducting elements 50 composed of one or more of thecarbides and borides referred to constitute vertically disposed barsimmersed by their lowor ends in the layer 51 of purified aluminumfloating on the body 52 of molten electrolyte, and attached at theirupper ends to a negative bus-bar 53. These elements may be connected tothe bus-bar by being brazed thereto (as at 54) or by having the bar 53cast about their upper ends. As in FIGS. 4 and 8, the exposed portionsof the elements 5'0 may be further protected against oxidation and otherharmful effects by a sleeve 55 or, alternatively, the sleeve may be casttherearound as will be described hereinafter.

FIG. 15 shows a modified arrangement of current-conducting elementswherein the element supplying the top layer 51, and designated as 56, isarranged in substantially the same manner as the lead for supplying thebottom layer 4-8, that is to say, it extends horizontally through theside wall of the cell and is connected at its outer end to an aluminumbus-bar 57 which is, in this case, connected to the negative pole of thecurrent-supply source.

With regard to three-layer purification or refining cells of the typementioned above (and described with reference to FlGURE 19) it may besaid that the insulating wall 46 defines a chamber having an upper zone(horizontal) adapted to receive a body of molten purified aluminum(cathode), a lower zone (horizontal) adapted to receive a body of moltenaluminum alloy (anode), and an intermediate zone adapted to hold acharge or body of molten flux or electrolyte.

As has been mentioned above it is desirable to protect thecurrent-conducting elements against oxidation and corrosion where theyare exposed for part of their length to oxidizing or corrosiveatmospheres as, for example, the current-conducting elements illustratedin FIG. 14. This is particularly so when the elements, cathodes, orextensions thereof are formed from the carbides, as distinguished fromthe borides such as titanium boride, the carbides being subject to aparticularly penetrating form of oxidation at temperatures of about 450C. at which a powdery non-protective oxidation product is formed. Asatisfactory method of protecting an element is to cover the exposedportion thereof with a sheath of aluminum. However, the mere applicationof an aluminum cover fitted mechanically over the lead, or even castaround it does not afford adequate protection.

Although the carbides and borides referred to can be wetted withaluminum in a vacuum at a temperature about 1150 C., or electrolyticallyby making the material the cathode in a reduction or refining cell,these methods are not convenient to use, particularly in the case oftitanium carbide, where it is a question of presheathing the element. Itis preferred, therefore, to coat the element with cobalt or nickel bysuitable means, to sinter the coating on to the surface of the material,and then to cast on aluminum under conditions such that it will alloywith the cobalt or nickel layer. This produces a firmly adherent outersheath of aluminum which is metallically bonded to the surface of theelement and which affords very good protection from oxidation.

As an example, the separate steps in the process may be carried out asfollows:

(a) A hot-pressed TiC bar 2" in diameter (to be used as acurrent-conducting element) is thoroughly cleaned to remove thegraphitic surface layer. This is most readily carried out bysand-blasting, but chemical cleaning (for example, in a hot alkalinepotassium permanganate solution) may also be used successfully. The barmust not be handled after cleaning.

(b) The bar is washed with Water and immediately plated with cobalt ornickel to a thickness of about 0.001 in. An ammoniacal cobalt sulphatebath is suitable for the cobalt plating and a nickel sulphate ammoniumchloride bath for the nickel plating.

(c) The plated bar is heated in a neutral or reducing atmosphere toapproximately 1050 C. and maintained at that temperature for about 30minutes to fire the cobalt or nickel onto the carbide. This treatment isconveniently carried out in an electric furnace provided with a hydrogenatmosphere, but good results have also been obtained by inserting thebar into a closed graphite container which is then heated to therequired temperature without any other precautions with regard tocontrol of 'the atmosphere.

((1) The bar is finally set in the bottom of an appropriate graphitemould contained in a steel shell and heated to 750-800 C. Moltenaluminum at the same temperature is then poured in and after a briefperiod at this temperature (say, minutes) the mould is removed from thefurnace and left to cool. The mould incorporates a large feeding head atthe top and the cooling of this is delayed by local heating so thatdirectional solidification of the metal towards the head is encouraged.

As a result a sound sheath A" thick open at one end and closed at theother is produced around the bar over practically the whole lengththereof (one end being left bare) and continues at its closed end as asolid rod (2%" in diameter) for approximately 10 ins. beyond the end ofthe TiC bar.

A complete sheathed electrode composed of titanium carbide and ready forinsertion into a three-layer cell is shown in FIGS. 16 to 18. The TiCbar 59 is 2 ins. in diameter and is normally about 9 /2 ins. long; itmay, however, be very much shorter than this if desired, and bars only 4ins. long have been operated successfully.

The aluminum sheath 60 is conveniently of the thickness mentioned above,i.e. inch, but this may be varied widely without affecting the efiicientfunctioning of the electrode. The purity of aluminum used is not veryimportant since only a small amount of it is finally dissolved in thecathode metal of the cell. Commercial purity (99.2%) aluminum is quitesuitable for the purpose.

- During the operation of casting-on the sheath the bottom end 61 of theTiC bar is inserted into a graphite jig in order to centralize the barin the mold. This portion of the bar (approximately /2 inch long) isthus not covered with aluminum. The electrode is normally immersed inthe metal layer in the cell, however, to a depth of about 2 inches, thelevel of the surface of this layer being indicated at 62 in FIG. 16, andit is important to ensure that the aluminum sheath is properly bonded tothe TiC to below this level in the casting-on operation. When in use ina cell, the sheath 60 on the bar 59 melts back to a point about /2 inchabove the level 62, and the skin of metal left behind by the adherent{sheath serves to protect the exposed interface from oxidation andattack by flux vapours.

Electrical connection from the external circuit is made to the end ofthe solid portion 63 of the sheath extending beyond the bar 59. This maybe conveniently done by flattening the end 64 of the solid portion 63and welding it directly to a suitable aluminum lead, e.g. a rectangularbar with a cross-section of 4 x 4 inches as shown at 65.

With regard to the above manner of sheathing the end ofcurrent-conducting element 59, it is to be understood that suchprocedure can be used as an alternative to that hereinbefore set forthfor affixing the cathodic currentconducting elements of reduction cellsto the bus-bars, e.g. affixing the element 10 to bus-bar 11 of FIG. 2 orafiixing the elements to bus-bars 26 of FIGS. 7 to 9.

In order to reduce losses of energy from the cell by radiation andconvection, it is advantageous to use a screen over the layer 62 ofcathode metal. This may be arranged to depend from the electrodesthemselves and a type of screen that may be used is shown in FIG. 16. Itconsists of an aluminum plate 66 approximately A" thick, which fits overthe sheath 60 and is supported by a collar 67; this can be adjusted toany given position the molten electrolytes used in the electrolysis.

and locked by means of the screws 68 which penetrate into the aluminumsheath.

One way in which electrodes of the type described above may be utilizedin a three-layer cell is illustrated in FIG. 19.

The main structure of the cell consists of a base and outer container70, constructed from refractory bricks of a material, such as magnesite,which is resistant to Iron conductor bars '71 are inserted through theside wall and are supported by the base of the cell; they are leftprojecting on the exterior of the cell so that convenient connection maybe made to the positive pole of the external electrical circuit. Theinternal floor of the cell is constructed of carbon or graphite blocks72, which are fitted over the conductor bars and are in good electricalcontact with them.

The molten bath itself consists of an anode layer 73 of aluminumcontaining a high proportion of copper to increase its density, anelectrolyte layer 74, and a cathode layer 62 of purified aluminum. Thetemperature of the bath under normal running conditions is usually inthe range 740-780 C.

Electrical connection is made to the cathode layer 62 by means of thesheathed electrodes described above. The TiC bars 59 are immersed in themolten aluminum to a depth of about 2 inches. The aluminum conductor 65,which forms the main suspension of each electrode, is clamped to themain external electrical bus-bar or cathode beam 75. These beams arenormally arranged so that they can be adjusted in height; thus when thelevel of the molten bath changes (for example, when the pure metal inthe top layer is tapped off) all the electrodes can be raised or loweredsimultaneously to correct their immersion. Individual adjustment of eachelectrode can be made by means of the clamp 76 used to hold theelectrode in good electrical contact with the beam.

Tests made on a cell constructed in the manner illustrated in FIG. 19embodying current-conducting elements 59 of titanium carbide illustratethe advantages of such a cell as compared to a corresponding orthodoxcell. The orthodox cell embodied nine 15 inch diameter graphiteelectrodes each carrying a current of 3,000 amperes and wherein thevoltage drop from the conductor to the purified aluminum layer 62 was0.4 to 0.6 volt. The modified cell embodied eighteen 2 inch diameterelectrodes each carrying a current of 1500 amperes and wherein thevoltage drop from the conductor 65 to the purified aluminum layer 62 was0.02 to 0.08 volt. The carbide elements required for the full equipmentof a cell were very much less bulky than the corresponding graphiteelectrodes due to their lower resistivity and this allows the electrodesto be arranged to considerable advantage with respect to shielding ofthe upper surface of the bath to minimize heat losses, and this aspectis of great importance in overall efiiciency of operation.

In order to minimize the loss of energy from the cell by radiation andconvection, it is beneficial to cover in the top with appropriatescreens. This can be done by a suitable combination of screens 66carried by the individual electrodes, and other screens 77 which rest onthe side walls of the cell.

It should be noted that the TiC bars are liable to crack if subjected tosevere thermal shock, and it is advisable to introduce them into thecell slowly, so that they are pre-heated before entering the moltenaluminum layer. Alternatively, a thermal barrier is provided by coatingthe bars with tar and then dusting them with carbon black. This delaysthe heating of the bars so that they may be inserted directly in thelayer of molten aluminum without any risk of cracking.

Although the above description refers to current-con-

1. A CURRENT-CONDUCTING ELEMENT CONSISTING ESSENTIALLY